I remember facing my second legendary hound in Pokémon gold, know that they'd run away/growl the first move and my first pokémon couldn't prevent it so I instantly tossed one of Kurt's balls and actually managed to catch it instantly, best childhood memory. I always wondered the % to actually manage to catch Suicune with a fast ball from full health.

the most amazing thing ive ever done was walking into the cave with mewtwo in it and forgetting i only had one weak ghastly and just said what the heck and through a ultra ball at it and caught it while it had full health nothing else that day could compare to the happiness i felt when that happened
if you read this i appreciate it and wish you all a good day

Legendaries are assholes I actually used a masterball on a zubat once just for the hell of it and it broke free and ran away. Yet I caught Raikou with a timer ball while he was sleeping. I think pokemon just likes to **** with you.

There' s always room for failures, that was one of them.
It's never happened since then as you said the chances are probably less than one.. I guess around 0.75% of missing since it is the best ball in the game.